What is mpg? How is it calculated?

Browse through any car reviews on carwow and you’ll almost certainly come across three letters: mpg.

You might have heard of it before, but what does mpg mean exactly, and why is important to you if you’re looking for a new car?

What is it?

The acronym “mpg” stands for miles per gallon. It tells you how fuel efficient a car is, and lets you compare cars’ efficiency levels.

It is a measure of how far a car is capable of driving while using a set quantity of fuel. In other words, when a car is capable of 40mpg, it states that if the car was filled with one gallon (4.55 litres) of fuel, it could travel for 40 miles before spluttering to a halt.

Alternative measures to mpg – converting from mpg from l/100km

Although we use mpg in the UK, imperial units like miles and gallons are rarely used in Europe (if ever), so fuel economy is measured by “litres per 100 kilometers” (l/100km) instead. As a result, when browsing some car manufacturers’ websites, you might see fuel economy displayed using these units. Without delving too deeply into mathematics, converting between the two figures can be achieved the following way:

Firstly, you need to know that 1mpg = 282.5l/100km. From there, if you know the car’s fuel consumption in l/100km, divide 282.5 by that number to find what the car can achieve in mpg. Likewise, dividing 282.5 by any mpg figure you have handy reveals a car’s fuel economy in l/100km.

Take this example: the Audi A1 fitted with the 1.4-litre TFSI petrol engine achieves a fuel economy figure of 5l/100km. To find its mpg figure we simply divide 282.5 by 5:

282.5 / 5 = 56.5 – therefore the A1 achieves 56.5mpg.

If you’d rather not calculate things yourself, there are several websites that can do the number crunching for you. Regardless, it’s worth remembering that high mpg numbers and low l/100km numbers signify a fuel efficient car.

In the United States, its measure of a gallon is not equal to the Imperial measure. As each Imperial gallon is equal to 1.2 US gallons, mpg figures on US market cars appear lower than they do here.

How is fuel economy calculated?

In Europe, all fuel consumption figures are measured the same way, regardless of which manufacturer’s car is being tested. The New European Driving Cycle (NEDC) was introduced to provide consumers with a fair comparison of all cars available for sale.

Four figures can be calculated from the test:

Urban fuel economy

Extra-urban fuel economy

A combined figure of the two

Carbon Dioxide emissions

Generally, the final two are the pair most people take pay attention to, but car brands will always provide all four in any official documentation.

The BMW i8 achieves 118mpg – but is that figure possible in the real world?

How is the mpg test carried out?

To ensure the fairest comparison possible, all cars are tested in air temperature between 20 and 30 degrees Celsius, and all engines must be started from cold. Buckle up, because you’re in for a wild ride…

Urban cycle

The test to mimic town or city driving is undertaken by accelerating and decelerating very slowly up to speeds of nine, 15, and 20mph respectively, a procedure which is repeated three further times. The four cycles total 780 seconds, and the car should have driven just over 2.8 miles in that time.

Extra-urban cycle

The extra urban test involves gently accelerating to 43mph, maintaining that speed for 50 seconds, decelerating to 30mph, maintaining that speed for 69 seconds, and then accelerating back up to 43mph.

Still awake? Good, because there’s more. After cruising at 43mph for another 50 seconds, the car must then accelerate to 60mph, hold that for 35 seconds, and then accelerate to 75mph, keep there for 10 seconds before slowly coming to a complete stop.

Is it a representative test?

Although these tests ensure an equal test for all cars involved, they skew results in favour of some cars more than others. In particular, hybrid vehicles benefit greatly, because the very gentle acceleration the tests require means that much of the cycle can be completed without ever using the petrol engine. That’s why cars like the BMW i8 and Mitsubishi Outlander PHEV are claimed to achieve incredible figures of 118 and 148mpg respectively, when in reality they will manage nowhere near that.

Save on average £3,600: Between January 2016 and March 2016 carwow buyers saved on average £3,600 off the RRP of car configurations.

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